1. Field of the Invention
This invention relates to a system for detecting abnormality in combustion of an internal combustion engine, and more particularly, to a misfire-detecting system which detects misfire occurring in a multi-cylinder type internal combustion engine based on fluctuations in the rotational speed of the engine.
2. Description of the Related Art
Conventionally, a system is already known from Japanese Provisional Patent Publication (Kokai) No. 61-258955, which detects abnormality in combustion, particularly so-called misfire, i.e. failure to ignite an air-fuel mixture, of a particular cylinder of a multi-cylinder type internal combustion engine, and warns the driver of the abnormality or inhibits fuel from being injected into the particular cylinder.
The conventional system is based upon the recognition that minor fluctuations are caused in the rotational speed of the engine by combustion in each cylinder. Therefore, an amount of change in the rotational speed of the engine per cycle of the fluctuations is obtained from the difference between the maximum rotational speed and the minimum rotational speed of the engine per each cycle of the fluctuations, and when the ratio of the amount of change in the present cycle of the fluctuations to the amount of change in the immediately preceding cycle of the fluctuations is smaller than a predetermined value, it is determined that a misfire has occurred in a cylinder.
However, the ratio can be small due to factors other than misfire, e.g. fluctuations in the air-fuel ratio of an air-fuel mixture supplied to the engine, or delayed flow of fuel adhering to the inner surfaces of the intake pipe at acceleration or deceleration of the engine, etc, which can cause the system to erroneously judge that a misfire has occurred.
Further, the ratio is obtained based on amounts of change in the rotational speed of the engine respectively calculated for two different cylinders which are adjacent to each other in the order of ignition. On the other hand, the amount of change varies between cylinders even under the same operating conditions of the engine. Therefore, the ratio can be small even when no misfire has occurred. This can also cause the system to erroneously judge that a misfire has occurred, even in the absence of the aforementioned factors other than misfire.